Proton exchange membrane (“PEM”) fuel cells are an established fuel cell technology being developed for many applications including transportation (e.g., in automobiles), portable power systems, or stationary systems (e.g., to power a home or building). High-temperature proton exchange membranes (“HTPEM”) are an emerging technology that allows a fuel cell to run at temperatures above 100° C., thus, among other benefits, providing improved tolerance of carbon monoxide and reducing the likelihood that water generated by chemical reactions in the fuel cell will collect in porous layers of the device.
FIG. 1 is a schematic representation of a typical HTPEM fuel cell 10, and FIG. 2 is an expanded detail thereof. The HTPEM fuel cell 10 includes a membrane electrode assembly (MEA) 12 that includes a central membrane 14, an anode 16 and a cathode 18. The central membrane 14, typically made of a polymeric material, allows passage of protons (H+) between the anode 16 and cathode 18 (i.e., it permits a proton current), but electrically isolates the anode 16 and cathode 18 from each other (i.e., it does not permit an electrical current between the anode 16 and cathode 18). FIG. 2 schematically presents a proton passing through the central membrane 14. One of the materials suitable for use between the anode 14 and cathode 16 of a HTPEM fuel cell is polybenzimidazole (PBI), which is a standard polymeric membrane material that is resistant to the high temperatures at which HTPEM fuel cells operate.
The anode 16 and cathode 18 each comprise respective gas diffusion layers 20, 22, which are porous so as to permit the passage of oxygen (O2, which may be supplied in air) supplied at the cathode-side gas diffusion layer 22 and a proton-donating fuel (in this example, hydrogen (H2)) supplied at the anode-side gas diffusion layer 20. The gas diffusion layers 20, 22 are also electrically conductive so as to permit the flow of electrons (e).
The anode 16 and cathode 18 further comprise catalyst layers 24, 26 (shown in FIG. 2 as comprising metallic platinum sites, such as platinum sites 28, plated on carbon particles, such as carbon particles 30, in a polymer matrix 32) are provided between each of the respective gas diffusion layers 20, 22 and the central membrane 14. The catalyst layers 24, 26 permit passage of protons (H+), and have pores 34 through which oxygen, hydrogen, or other gases can pass. The catalyst layers 24, 26 are in electrical contact with the gas diffusion layers 20, 22 at their interfaces 36, 38. Each catalyst layer 24, 26 is flat, yet it must be of sufficient thickness to present a sufficient number of active catalyst sites (e.g., platinum sites 28) to provide a sufficient electrical current.
At the catalyst layer 24 of the anode 16, hydrogen gas is catalytically converted to protons and electrons. The central membrane 14 allows passage of protons through the central membrane 14 to the cathode-side catalyst layer 26, where they react with oxygen to form water. Since the central membrane 14 is not electrically conductive, electrons do not pass through it, but pass through a external circuit 40 to the cathode 18, where they participate in the reaction between the protons and oxygen. The three species (O2, e−, and H+) must converge on a catalytic site (e.g., a platinum site 28) in order to complete the electrochemical reaction that generates electricity in the fuel cell 10.
As is apparent from FIGS. 1 and 2, and from the foregoing discussion, conventional HTPEM fuel cells, such as HTPEM fuel cell 10, can be considered to be a stack of disparate materials (e.g., gas diffusion layers 20, 22, catalyst layers 24, 26 and central membrane 14) connected by flat, two-dimensional interfaces. Consequently, electron current densities, and, more importantly, proton current densities, are limited, in part, by the bulk properties of the layers and the cross-sectional areas of the interfaces. Diffusion mass transport through the layers is similarly limited. The catalyst layer 26 serves as the location of “triple phase boundaries,” or regions in the cathode 18 in which protons passing from the anode 16, oxygen entering from the cathode 18, and electrons from the external circuit 40, must all converge in order to complete the electrochemical reaction. Flow through the catalyst layers 24, 26, hence, must facilitate the transport of all of these species. To the extent that the resistance to these flows can be reduced, improved performance of the HTPEM fuel cell 10 can be achieved.